Would Polish forces resist a Soviet or Warsaw Pact invasion of their homeland with all their legendary tenacity, or would they wilt before it? More important , would they take part in an assault on Western Europe if the Soviet Union launched one, or would they refuse to fight?

Like the Western intelligence agencies that observe Poland with an uncommon zeal these days, neither Defense Minister Ustinov nor Marshal Viktor G. Kulikov, the Warsaw Pact's commander in chief, can be certain what its forces would do, though they presumably have a shrewd idea.

The Polish Embassy here insists that the heady liberalization of the past year has not affected the fighting spirit of Polish forces. But a leading US authority on the Warsaw Pact, who asked not to be identified, contends that their morale "has to have deteriorated."

Indeed, he recently was informed by a reliable Polish source that 20 to 25 percent of Poland's armed forces are members of Solidarity, the country's giant independent trade union. Support for Solidarity, which so far has effected a peaceful revolution in the country, is even apparent among the Soviet officer corps, he adds.

It would be impossible to insulate Polish forces from a nation seething with dissent when 185,000 of them are conscripts, experts agree. The men in uniform, says Richard Davies, who served as US ambassador to Poland from 1972 to 1978, are "the sons and nephews and brothers of all the fellows demonstrating. They're very conscious of what's going on."

That support for Solidarity exists in the military appears to be borne out by other experts on Poland.

Tadeusz Szafar, a visiting Polish scholar at Harvard's Russian Research Center, reports that, according to one of his sources, Polish Army counterintelligence was very anxious to weed Solidarity sympathizers out of the Army after last year's big strikes. "I think it might be true," he says. "It makes sense."

According to his contact, moreover, workers at defense plants have been fired for their membership in the union, although dismissals of this kind have been officially blamed on economic stringency, he says.

Solidarity's appeal does not seem to be confined to the regular forces. The country's 350,000-strong citizens' Militia is thinking of joining the union, says Aloysius Mazewski, president of the Chicago- based Polish-American Congress. Mr. Mazewski recently returned from a trip to Poland.

While disaffection in the ranks of Poland's military machine might ensure that any Russian invasion of the country was met with stiff resistance, it could also have dire strategic implications for Moscow.

"For a Soviet attack on NATO to be effective, participation of the non-Soviet Warsaw Pact forces -- particularly . . . those of Czechoslovakia, the German Democratic Republic, and Poland -- would be essential," declared a Congressional Budget Office study last year.

Without their cooperation, Moscow's ability to strike with little or no warning "would be significantly degraded," it observed, "as would the Pact's overall combat capability throughout the 90-day period following mobilization."

Might Poland desert the pact if it had to participate in an attack on the West?

Any member of the East-bloc alliance who felt an assault on NATO ran contrary to its interests "could present problems for the Soviets in combat operation," declared Warsaw Pact experts Dale Herspring and Ivan Volgyes in an article published in "Armed Forces and Society" last year.

"From Moscow's standpoint," they observe, "to be highly reliable in an offensive campaign, the populations of Eastern European countries involved would have to identify their own national interests closely with those of the USSR." And they add: "As recent events in Eastern Europe have shown, under most circumstances, such a development is unlikely."

The authors note that Poland's traditional foes in the West are the Germans. "Assuming that Moscow was able to convince the average Polish soldier that a preemptive attack was necessary in order to stave off a German invasion, the Poles would probably fight gallantly," they observe. But they emphasize that the Poles have "deep bonds of friendship" with Britain, France, and the US. "Thus their willingness to engage in combat operations against NATO forces in anything but a highly successful engagement is questionable at best."

Messrs. Herspring and Volgyes are not alone in doubting that Polish forces would display blind obedience to Moscow. "I don't believe for one minute the Soviets are going to attack Western Europe," says Ambassador Davies, "but if the Polish people felt that a Soviet attack [on NATO] constituted unprovoked aggression, Moscow would have a lot of trouble with the Polish Army."

If Poland refused to join a Warsaw Pact invasion of Western Europe or if it pulled out when an attack bogged down, Moscow would lose a powerful ally.

Apart from its 15 divisions (with their 3,600 tanks and 7,500 armored vehicles), Poland can field some 700 combat aircraft and a variety of naval vessels. It has always received the pick of Soviet military equipment, and in the view of some experts its Air Force is the most technically advanced in the Pact -- with the exception of the Soviet Union, that is.

But Poland brings more to the Warsaw Pact than its considerable military might. It also brings its crucial strategic location.

Not only does the country dominate the huge plain that stretches from the English Channel to the Urals, but also it carries vital road and rail links between the Soviet Union and East Germany, where Moscow maintains 19 divisions ( 9 tank and 10 motor rifle) in its so- called Group of Soviet Forces in Germany (GSFG). In addition, the "Druzhba" oil pipeline crosses Poland on its way to East Germany from Siberia.

Should Poland become thoroughly disaffected -- either by reason of a Soviet invasion or a general war with the West -- Moscow could expect such road, rail, and fuel links to be the targets of sabotage. The security of Poland's 62 military airfields, moreover, also might be in doubt.

Elements of two to three Polish divisions based at Legnica in southwest Poland are charged with securing the lines of communication between the GSFG and the Soviet Union's Baltic, Byelorussian, and Carpathian Military Districts. But can the Soviet Union rely on them? Some experts maintain that Moscow would need to move 30 divisions into Poland to ensure that supplies and reinforcements could traverse it in safety. At the moment, two Soviet tank divisions are stationed in Poland.

As Ambassador Davies sees it, Poland is "absolutely critical" to the Soviet Union.

Aloysius Mazewski agrees. "Poland is very vital to the Soviets. If it broke away, East Germany would be completely isolated," he says. Mr. Szafar adds: "There's no way the Soviets could give up Poland without giving up their positions in East Germany."

A secure Poland would be of the utmost importance to the Soviet Union if it were to engage NATO on the Central Front in any general war, military observers assert.

"Military success in the Central Region would defeat the loser's best armies, ensure control of the economic and political power of Western Europe, and allow the winner to control access to the Soviet Union," declared last year's Congressional Budget Office study.

Besides its strategic importance, Poland also performs something of a cordon sanitairem function for Moscow. Bluntly, it helps keep West Germany at arm's length.

Except for its broad rivers, marshland, and lack of all-weather roads, the western Soviet Union offers little to hinder an invader -- as the Nazis demonstrated when they ripped into the country in 1941. "The Russians are afraid of the [West] Germans," Mr. Mazewski says. "They want a big stretch of land between them."

When considering whether to invade Poland, the Soviet Politburo has no doubt weighed up the strategic perils of letting the country pursue a path of liberalization.

If Moscow intervenes, would the Poles fight?

"The probability of military resistance to an external threat will increase to the degree to which the country concerned is being invaded by one of its traditional enemies," declare Herspring and Volgyes.

"The Poles continue to be willing to offer determined resistance to invasions from any of their traditional enemies," they assert -- one of which is Russia.

Poles have not forgotten that Stalin invaded their country in 1939 (when Hitler had virtually crushed it) and in accordance with the Nazi-Soviet Pact seized a large tract of its eastern territory.

Neither have they forgotten that Soviet executioners shot some 4,000 officers and men of the Polish Army in Katyn forest near Smolensk in 1940 after they surrendered to the Red Army. A further 11,000 officers and men who gave themselves up at the same time never have been accounted for.

If Moscow decides to bring the Poles to heel with East German troops (regarding them, as it does, as extremely reliable and superbly trained), Poland's fury would know no bounds, say Western experts. Perhaps no other country -- with the exception of the USSR -- suffered so grievously under Nazi rule as did Poland in World War II.

The 1968 invasion of Czechoslovakia (carried out by Soviet, East German, Bulgarian, Hungarian, and Polish forces) showed that Pact members were not averse to interfering with another member's internal affairs, as Eastern European military authority A. Ross Johnson pointed out in a 1976 paper. But it did not indicate, he stressed, that they could "contribute effectively to military operations against one of their number."

The Polish forces that took part in the Czech operation acted with "sullen compliance" and "were deeply ashamed of what they were doing," says Ambassador Davies.

Whether the Soviet Union (with or without its allies) will invade Poland and whether the Poles would fight back are questions that continue to divide experts.

Informed sources in London seem to feel that the vast majority of Polish troops would not immediately commit themselves in the event of a Soviet invasion.

"The chances of Polish Army resistance are low," declares a Warsaw Pact expert in Washington. "But the chance of resistance by Polish soldiers is very high."

Ambassador Davies says he has "a strong belief that the great majority of Polish troops would side with the people. The question is would it be at a division level or regiment level?"

By some accounts the Polish officer corps is decidedly pro-Polish. But Mr. Szafar claims that the "commanding heights of the Army are totally loyal to the Russians." Observers point out that at these exalted levels, officers are all members of the Communist Party and have, without exception, attended Soviet military academies. Nevertheless Mr. Szafar concedes, "There might be surprises."

In Ambassador Davies's view the Soviets will not storm into Poland "unless the government and party totally lose control." He says the coming winter will be a "critical time" with inadequate coal supplies adding to hardships. "The index of misery will rise quite a bit," he predicts. The ambassador admits that the Soviets can occupy Poland at will. "But what do they do then?," he asks.

He believes that the current "disorder" in Poland will continue "until the government talks serioulsy with Solidarity about a comprehensive economic reform program -- which it has not yet done."

If Moscow decides to invade Poland, the Pentagon believes it would know a week in advance of the operation. Such telltale signs as increased radio communications, vehicular traffic, equipment maintenance, driver training, and range-firing could not be concealed from agents, satellites, and monitoring equipment, says a spokesman.

But this may be an optimistic claim. NATO was caught napping when the Warsaw Pact invaded Czechoslovakia. Pact forces lingered near the unsuspecting country after completing maneuvers, then swooped. Surprise and deception are hallmarks of Soviet military strategy, observers note.

All in all, the Kremlin probably has few illusions about the reliability of Polish forces in any situation. Though well-equipped, they always have been denied weaponry that might pose a threat to the Soviet Union. According to the co-authors of "Soviet Air Power," Bill Sweetman and Bill Gunston, the Polish Air Force has no strategic aircraft and no long-range attack machines. In fact, it has no modern Soviet aircraft other than the swing-wing Su-20.

Military experts claim that the Soviet Union never has really been able to count on Poland militarily. They cite the appointment in 1949 of Konstantin Rokossovsky, a Soviet marshal, as Polish defense minister.

When Poles find their vast eastern neighbor particularly threatening, many hark back with pride to Aug. 15, 1920, when Marshall Jozef Pilsudski defeated the Bolsheviks at the gates of Warsaw, saving the country and stopping the westward advance of communism.